Optically transmissive components used to transmit and redirect light from point light sources are widely used. Back lit panels for LCD displays are a common example. These mechanisms redirect light from a point light source along the panel by positioning a light source so that its light rays are transmitted in the panel by internal reflection. The light sources typically employed for the back lighting of LCD displays are small light-emitting diodes (LEDs). Recent advances in LED technology have increased the light output and efficiency of these light sources. LEDs are now employed for many directional lighting applications such as reading lamps, flashlights, headlamps, etc. LEDs are point light sources emitting light over a range of angles surrounding an optical axis. The directional light of an LED is easily adapted to directional lighting applications. However, non-directional lighting such as that provided by a fluorescent tube has heretofore been difficult to achieve using LEDs.
U.S. Pat. No. 4,422,719 to Orcutt discloses a light pipe with a translucent coating over a core material which allows the light transmitted axially to be dispersed radially around the light pipe. The translucent layer causes the entire width of the light pipe to be illuminated.
U.S. Pat. No. 4,466,697 to Daniel discloses another type of light pipe consisting of an extruded material including co-extruded reflective particles. These particles randomly reflect light transmitted in the light pipe so it emerges in a non-directional pattern. The light emitted from this prior art light pipe is random and consequently the entire light pipe is illuminated. U.S. Pat. No. 4,195,907 to Zamja et al employs dispersed air bubbles in an extruded optical fiber that function similarly to the reflective particles of Daniel.
U.S. Pat. No. 3,535,018 to Vasilatos discloses an optical fiber that has notches cut into the fiber to provide reflective surfaces. Light is emitted from the fiber wherever the notches interrupt the outer layer of the optical fiber. The notches are positioned in a random manner, causing the entire fiber to be illuminated.
U.S. Pat. Nos. 6,031,958; 6,160,948 and 6,337,946 to McGaffigan disclose optical light pipes with a particular pattern of reflecting surfaces arranged to produce a light emission pattern that looks like a laser is traversing the length of the light pipe. McGaffigan discloses hollow light pipes in which the inside surface of the light pipe is formed into a plurality of prismatic surfaces. The prismatic surfaces define a curved surface either on the inside or the outside of the prismatic element. The prismatic surfaces redirect light rays in a plurality of planes that are perpendicular to a tangent to the curved surface defined by the prismatic elements. This arrangement of optical elements creates an illusion that light is emitted from a centerline of the light pipe, when the light is actually emitted from an exterior surface of the prismatic element. The McGaffigan patents disclose a particular pattern of prismatic elements calculated to produce the desired laser-like light emission pattern. This laser-like light emission pattern may not be the most efficient or least costly to manufacture for an illumination system.